I guess you're looking at winners and losers. In terms of deaths then almost certainly Stalin is the greater evil - 20M dead as opposed to 10M. But these numbers are meaningless - every single one of them was someones parent or child and they all died because of an idea or an ideal.
In the west, Hitler is seen as the bigger evil since it's only really over the past 10 years or so that the true scale of the War in the East and the brutality of it has become more widely known.
I'd have to say that Hitler was the truly evil one. Stalin, whatever his political purges and internal machinations and suppression in Russia before 1941, caused most of his deaths as a reaction to what Hitler did - yes, he sent people to their deaths in great waves, yes his incompetence as a general caused huge losses, yes he was paranoid and ignored good advice and intelligence (even to the starting date for Barbarossa) but it was all, essentially, because he was invaded when he wasn't ready for it and certainly wasn't geared up to respond and he had to do something.
Hitler on the other hand made it clear from very early on that he was after the Jews, even if the industrial death camps weren't there right from the start (the concentration camps were there pre-war, but then us British invented those, I believe, in the Boer War). So he intended extermination of the Jews - and Gypsies, Communists, Homosexuals right there. His evil extended to making the German Wehrmacht & SS his instruments - read about what the 'ordinary' soldiers did during Barbarossa and later and see if you can believe them when they say they didn't know what was going on. Atrocity became, at Hitlers instigation, the method of war that has been popular amongst tinpot dictators and bullies worldwide.
It's all a bit pointless in one way to consider one worse than another - there could have been upwards of 50M people killed in WW2, no-one will ever know for sure. The important point is have we learnt from it and what would YOU do if you had total power of life and death over helpless people? The first answer is NO, the second might make you uncomfortable.....